Electric furnace.



J. HKRDEN.

ELECTRIC FURNACE.

APPLICATION FILED MAY '7. 1909v Patented New 23, 1910.

Hmeaoeo. M

.rorrANNEs HARDEN, or LoNnoN, ENGLAND.

ELECTRIC FURNACE.

erases.

Specification of Letters Patent.

Patented Nov. 29,1910.

Application filed May 7, 1909. Serial No. 494,717.

To all whom it may concern:

Be it known that I, JoHANN-ns HZRDriN, a subjectof the King of Sweden, and res ding in the city of London, England, have invented a certain new and useful Improvement in Electric Furnaces, of which the following is a specification.

My invention relates to electric furnaces and particularly to furnaces of the induction type, theobject of my invention=being to provide an improved a paratus of this character in the particu ars hereinafter pointed out. p

In induction furnaces of the common type the crucible or lining of the chamber in which the charge is contained, is of refractory non-conducting material and it is consequently necessary in order to start the melt of the charge, to first introduce cast or welded rings of the material to be treated. The induced current flowing through these rings raises their'temperature by reason of the resistance encountered therein, until they and the adjacent portions of the charge are melted and the flow of the current through the charge thus established.

In addition to the expense attendant uponthe preliminary casting or welding of the rings, this process is very unsatisfactory since it is ditficult to secure rings of the same analysis as that of the rest of the charge. Furthermore it is practically impossible to obtain a ring of absolutely uni form resistance. Hence it is very apt to melt at one or two spots of high resistance and thus break the secondary circuit before a successful melt can be efi'ected. Consequently it'is necessary to keep a supply of molten metal at hand ready to be poured into the furnace to complete the rin should this be broken as described. This additional drawback is enhanced by the fact that it renders the induction furnace unsuitable for. use in the production of certain alloys, the proportions of which must be exactly maintained.

A further and even more serious disadvantage results when dealing with metals or alloys of high conductivity such for instance as copper or bronze. In this case the current density may, owing to the low resist ance of the charge, become so excessive that the so-called pinch efiect takes place, viz.

the elementary paths of current through the metal, owing'to the high current density, attract each other th such force that the a refractory material such as ,WllOlG charge is contracted into a narrow stream of metal which is immediately vaporized by the current, thus breaking the ring. separated portions of metal will again flow together, causing a rush of current, and the same play begins again. This characteristic makes it almost impossible to treat cop er,

for instance, in a commercial way in in uction furnaces. The present invention prorides a furnace in which these disadvantages are eliminated.

In the'accoinpanying drawings, Figure 1 is a cross section of a'furnace in which my; nvention is incorporated; Fig. 2 is a vertb cal section of a crucible of modified or n; Fig. 3 is a similar view of another m'o cation.

My improved furnace has a lining on" crucible composed ofan intimate mixture of a suitable conductor such as aphite, with re clay which, will stand the heat'of the melt without material injury, the admixture being iii such proportions as to secure a certain amount of electric conductivity withdut a material lowering of the refractory property of'the clay. When a. crucible of this type is placed around the primary coil and magnet core of an in duction furnace, and the coil energized, a secondary current will be induced in the wall of the crucible, the resistance of which will heat the crucible, and eventually the charge, sufliciently to establish a melt. In-

asmuch as the current will be distributed in accordance with the section of the crucible wall, I propose to make the latter of reater thickness in the body of the crucibl e than at the lip and thus secure its gradual heating from the bottom up as the electric energy is transmuted into heat. In combination with a crucible of this type, tlie,,resistance of which is high, I propose to use-1a divided rimar Y coil in order to increase the induc- P v l tive e ect of the ma net core duringhhe heating up of the crucible. Thus during theperiod of heating up, the full voltage of'the furnace is sent through a portion, say only i withtlie result a third, of the primary co that the magnet is greatly overexcitegi; and its inductive effect greatly increased in'order 'toheat up the crucible in the shortest tiniei possible. As soon as the charge in the crucible has been melted sufiicientlyto form 'a ,complete ring (which is manifested by a rise .of t '0 primary cur en thiwhole M l-1S" After the break has occurred the 60 switcl'ied in and the melt is completed with .the current. passing through the entire pri mary .coil. This switching in of the ren-iaiudcr of the coil may be eflccted either by hand or by means of an automatic switch of. the overload type which is actuated through the sudden rise of the primary current.

In some cases it may be desirable to so proportion the graphite and clay (or their equivalents) that the resistance of the crucible is n-uch higher than that; of. the charge, with the result that after the melt has-begun practically the entire induced current flows through the charge. In other cases, for instance when dealing with a charge of brass or aluminum, it may be desirable to make the proportion such that even after the charge is melted and the current is passing through the same, an ap' 'ircciable percentage of the current is still passing through the crucible and generating heat. This-result might also be attained by suitably'regulating the cross section of the crucible with relation to the cross section of the metallic bath.

For the treatment of certain metals, a crucible of graphite and clay would be unsuitable as this would inevitably lead to the introduction of'both carbon and silicon as impurities into the charge. \Vhen desirable therefore, I propose to rovide the crucible with aneutral facing l ig. 2) which may vary with the charge treated. Thus for a charge of aluminum, the graphite crucible might be suitably faced with magnesite. In some cases when a neutral facing is neces sary, it however sutlicicnt to employ a crucible which has already been in use, since the graphite in the surface adjacent the charge will, for the most part, be taken. up by the/firstfew charges, leaving only a thin facing or the refractory clay as a substan tially neutral facing.

A furnace of the character described has the obvious advantage that a melt may be -lttt'lttl with the charge in any shape of met at, granular, irregular or otherwise, without the aitlol' a ring or a molten charge. Further the daliger of pinching is elimi uatt-ll, because as soon as the charge shows a tendency ,to contract, the contracted portion ot' the metal is bridged over by the conductive. walls of the crucible which, being solid, cannot: contract, thus checking this plu-noimmou very successfully. Furthermore a lining of this character, being in practice a cou'ipressed crucible made in one 'iiecc, through the use of a suitable binder, permit of shuttix'ig down the furnace and allowing the same to cool over night without danger to the crucible. This is impossible .with linings of the character hitherto in use which are stamped in the furnace from powdery material which invariably cracks on cooling. Again, the present furnace may be i completely emptied after each melt since there is no necessity for leaving a remainder of one charge in the crucible to facilitate the starting of the next charge. This is a'distinct advantage when treating charges of different quality.

Obviously the crucible may be altered to meet the demands of the particular metal treated, and may be varied in detail of cons'truction without. departing from my invention and I do not limit myself to the precise structure shown. Thus in Fig. 3 for instance, I have shown merely a ring of refractory crucible of the common type, instead of forming the entire crucible in this manner.-

Iclaim as my invention:

11 In an induction furnace for melting metals, a crucible in the walls of which the induced current flows untila melt is established, said walls comprising an intimate able conductor. I r

2. In an-induction furnace for melting metals, :1 crucible in the walls of which the induced current flows until amelt is estabmixture of a refractory material and a suitable conductor in such proportions that upon a contraction of the melted charge at any point the current flows at that point through the Walls of the crucible, thereby avoiding the pinch effect mentioned, substantially as described.

3. In an induction furnace for melting metals, a crucible having walls comprising an intimate mixture of refractory material and a suitable conductor in. suitable proportions, said Walls being of greater cross section in the body portion than at the lip, whereby a gradual heating of the charge from the bottom up is secured upon the passage of the induced current through said walls, substantially as described.

4. In an induction furnace for melting metals, a crucible having conducting Walls through which the induced. current passes during the establishment of a melt,,.said

capacity in the body portion than at the lip, for the purpose described.

5; in an induction furnace, a melting chamber or crucible of relatively high-com ductivity. said crucible being of greater cross section in its body than at its lip, in combination with a. divided primary winding adapted to be partially energized to inat the starting of the furnace to secure a preliminary melt; and means for switching in a further length of said primary winding after av melt has been established.

graphite and clay molded in the WtIll Of a mixture of a refractory material and a suit-- lished, said Walls comprising an intimate walls bcing of greater current carrying.

duce a strong current through said crucible G. In an induction urnace for mclting metals, a melting chamber having in its structure a mixture of refractory and conducting materials forming a secondary circuit; with a primary winding for inducing a current in the structure of said melting chamber and in the material contained therein.

7. In an induction furnace for melting metals, a melting chamber having in its structure a relatively high resistance mixture of refractory and conducting materials forming a secondary circuit, with a primary winding for inducing a current in the structure of said melting chamber and in the material contained therein.

8'.- In an induction furnace for melting metals, a melting chamber composed of an intimate mixture of an electrical conductor and a refractory material constitutin a secondary circuit, with a primary win ing for inducing a current in the structure of said chamber and in the charge contained therein.

,9. In an induction furnace for melting metals, a melting chambert having in its structure a mass of conducting "material forming a secondary circuit and provided charge with a neutral facing for keeping a free from contamination from the material of the chamber; with avprimary winding for inducing current in the structure of said melting chamber and in the material contamed therein.

10. In an induction furnace for melting.

metals, a melting chamber having in its structure a mixture of refractory'and conducting materials forming a secondary circuit; with means for first over-exciting of the magnet of the furnace and th reafter automatically reducing the inductive effect of said magnet when the furnace has been heated to a predetermined point.

11. In an induction furnace for melting metal la Inciting chamber having in its structure a mixture of refractory and conducting materials forming a secondary circuit, in combination with a divided primary windingadapted to be partially energized to produce a higher voltage in the induced circuit at the starting of the furnace to secure the melt of the charge and means for switching in a further length of said primary winding after amelt has been established, for the purpose described.

In testimony whereof ,I have signed my name to this specification, in the presence of two subscribing witnesses.

JQHANNES HARDEN. 

